Many boats and other watercraft are propelled by one or more inboard or outboard engines or a stern drive system, which drives one or more propellers. Each propeller typically has three or four blades, but may have as few as two or as many as five or six. The base of each blade is mounted at an angle, or pitch angle, relative to a radial plane transverse to the axis of rotation of the propeller shaft. Propellers may be constructed with blades having a fixed pitch. The fixed pitch is typically when the propeller blades are fixed at a pitch angle that provides maximum efficiency at a designated operating condition. Fixed pitch propellers typically have a reduced efficiency at operating conditions other than those designed for. Alternatively, the pitch of a propeller can be fixed to provide better acceleration or pulling capacity at lower speeds, which typically results in a reduced top speed. As a result, fixed pitch propellers typically are a compromise between acceleration, a boat velocity and fuel consumption.
One way to improve the efficiency of propellers at most speeds is to provide a propeller with blades having a variable pitch angle. This type of propeller has therefore a variable pitch and is called variable pitch propeller. One example of a variable pitch propeller is described in U.S. Pat. No. 6,896,564, which is incorporated herein by reference in its entirety. FIG. 8 shows typical blades 300′ of a variable pitch propeller 54′. The blades 300′ are pivotally connected to a hub 94′ for rotating about their pitch axis 102′. An actuator (not shown in that figure) is linked to the blades 300′ to pivot them between a first pitch angle and a second pitch angle at which actuation of the propeller assembly 54′ produces forward thrusts of different strength (positive pitch). In a few cases, the first pitch angle produces a forward thrust, and the second pitch angle produces a rearward thrust (negative pitch) to the watercraft.
To avoid interferences between adjacent blades 300′ during pivoting, the blades 300′ usually have a smaller chord 347′ at the hub and a smaller tip area 301′ than the blades mounted on typical conventional outboard engine fixed propellers. However, a longer chord at the hub and a larger tip area are desirable as they maximize the available surface area of the blade and thus create a greater thrust, and favour flow control. Controlling the flow of water at the tip of the blade, and especially at the trailing edge, is critical for minimizing cavitation and losses when operating at high rpm.
In addition, the blades of conventional variable pitch propellers are secured to the hub of the propeller by one or more screws inserted into threaded apertures located at the base of the blade. The threaded ends of the screws are engaged in the blade, while the heads of the screws are in the hub. That way the screws are not apparent on the blade. While this arrangement does secure the blades to the hub, it makes it difficult for the user to replace a damaged blade from the propeller. To detach the blade from the hub, the user has to first disassemble the hub in order to access the screws, before being able to unscrew the blade from the hub.
Therefore, while conventional variable pitch propellers are adequate for their purposes, there is nonetheless room for improvement in the art.